1. Field of the Invention
This invention relates to illumination and projection systems that collect and condense light spread over a wide collection angle to a small target.
2. Description of the Related Art
The objective of systems that collect, condense, and couple electromagnetic radiation into a target such as a standard waveguide, e.g. a single fiber or fiber bundle, or output electromagnetic radiation to the homogenizer of a projector, is to maximize the brightness of the electromagnetic radiation at the target. There are several common systems for collecting and condensing light from a lamp for such illumination and projection applications.
U.S. Pat. No. 4,757,431 (xe2x80x9cthe ""431 patentxe2x80x9d), the disclosure of which is incorporated by reference, describes a light condensing and collecting system employing an off-axis spherical concave reflector to enhance the flux illuminating a small target/and the amount of collectable flux density reaching the small target. Another light condensing and collecting system is provided by U.S. Pat. No. 5,414,600 (xe2x80x9cthe ""600 patentxe2x80x9d), the disclosure of which is incorporated by reference, describes the use of an ellipsoid concave reflector. Similarly, U.S. Pat. No. 5,430,634 (xe2x80x9cthe ""634 patentxe2x80x9d), the disclosure of which is incorporated by reference, describes the use of a toroid concave reflector.
The systems of the ""431, the ""600 and the ""634 patents provide a near 1:1 (unit magnification) image and conserve brightness from the light source. However, these systems lose their 1:1 (unit) magnification, thus degrading overall projection system performance, as the collection angle of the reflector is raised to increase the amount of collected light. Therefore, in these systems, increasing the collection efficiency decreases the quality of the produced image.
To address problems in the known optical collection and condensing systems, U.S. patent application Ser. No. 09/604,921, the disclosure of which is incorporated by reference, provides a dual-paraboloid reflector system that is advantageous in many respects to other known systems, including the achievement of 1:1 magnification for small-sized light sources.
This optical collection and condensing system, as illustrated in FIG. 1(a), uses two generally symmetric paraboloid reflectors 10, 11 that are positioned so that light reflected from the first reflector 10 is received in a corresponding section of the second reflector 11. In particular, light emitted from a light source 12, such as an arc lamp, is collected by the first parabolic reflector 10 and collimated along the optical axis toward the second reflector 11. The second reflector 11 receives the collimated beam of light and focuses this light at the target 13 positioned at the focal point.
To facilitate the description of this optical system, FIG. 1 includes the light paths for four different rays (a, b, c and d) emitted from the light source 12. The light output from an arc lamp subtends a cone angle of about 90xc2x0 when viewed in a direction normal to the lamp axis, as shown in FIG. 1(a). The light output from an arc lamp subtends a cone angle of nearly 180xc2x0 when viewed in a direction parallel to the lamp axis, as shown in FIG. 1(b). Rays a and d indicate the extents of the cone angle.
The optical system of FIG. 1 may employ a retro-reflector 14 in conjunction with the first paraboloid reflector 10 to capture radiation emitted by the light source 12 in a direction away from the first paraboloid reflector 10 and reflect the captured radiation back through the light source 12. In particular, the retro-reflector 14 has a generally spherical shape with a focus located substantially near the light source 12 (i.e., at the focal point of the first paraboloid reflector) toward the first paraboloid reflector to thereby increase the intensity of the collimated rays reflected therefrom.
One shortcoming of the above described dual-paraboloid optical system is that a large input angle is produced, resulting in numerical apertures as high as 1.0. As a result, some of the rays strike the target 13 at high angles of incidence relative to the target surface. Such high angles of incidence produce Fresnel reflections that introduce losses.
In U.S. application Ser. No. 09/669,841, the disclosure of which is incorporated by reference, a dual ellipsoidal reflector system is described as providing 1:1 magnification for small light source target. This optical collection and condensing system, as illustrated in FIG. 2, uses two generally symmetric ellipsoid reflectors 20, 21 that are positioned so that light reflected from the first reflector 20 is received in a corresponding section of the second reflector 21. In particular, light emitted from the light source 22 is collected by the first elliptical reflector 20 and focused onto the optical axis 25 and diverged toward the second reflector 21. The second reflector 21 receives the divergent beam of light and focuses this light at the target 23 positioned at the focal point.
As may be seen in FIG. 2, the dual-ellipsoid system suffers from the same disadvantage as the dual-paraboloid system in that some rays strike the target at large angles of incidence, producing Fresnel reflections. But as with the systems described above, Fresnel reflections caused by the large collection angle introduce losses.
Another embodiment of the dual-ellipsoid system may be seen in FIG. 3. This dual-ellipsoid system suffers from the same disadvantage as the above-mentioned dual-paraboloid and dual-ellipsoid systems in that some rays strike the target at large angles of incidence, also producing Fresnel reflections.
A tapered light pipe 40 with a flat input surface 42 for use with the above systems is shown in FIG. 4. Rays of light axe2x80x2, bxe2x80x2, cxe2x80x2, and dxe2x80x2 reflected by second reflector 41 converge at flat surface 42 of tapered light pipe 40 at large angles of incidence as shown in FIG. 4. The tapering of light pipe 40 will transform the large input angles into smaller output angles. The degree to which the angles are transformed will depend on the degree of taper. The output angles are designed for a specific system by matching the output device to the light pipe. As shown in FIG. 4, the input angle at input surface 42 of the light pipe 40 between rays axe2x80x2 and dxe2x80x2 can approach 180 degrees, i.e. an angle of incidence of 90 degrees. Such a high angle of incidence will introduce high losses due to Fresnel reflections. For uncoated light pipes made with glass or quartz, the Fresnel reflection loss becomes very significant at angles of incidence larger than about 75 degrees.
Therefore, there remains a need to provide a method of coupling light from a small source to illumination and projection systems with reduced losses due to Fresnel reflections.
An optical coupling element for use in large numerical aperture collecting and condensing systems. The optical coupling element includes a lens with a center and a curved surface. The optical coupling element is placed substantially at the input end of a fiber, fiber bundle, or homogenizer. The curved surface reduces the angle of incidence of the light striking the input end of the optical coupling element such that the Fresnel reflection is greatly reduced.
In particular, a collecting and condensing system comprises a source of electromagnetic radiation, and an optical coupling element to be illuminated with at least a portion of the electromagnetic radiation emitted by the source. The optical coupling element comprises a lens and a tapered light pipe, the lens having a center and a curved surface distributed about the center. A first reflector having a first optical axis and a first focal point is arranged facing substantially symmetrically a second reflector having a second optical axis and a second focal point on the optical axis, such that the first and second optical axes are substantially collinear. The source is located substantially proximate to the first focal point of the first reflector to produce rays of radiation that are reflected by the first reflector towards the second reflector, converging substantially at the second focal point. The center of the lens is located substantially proximate to the second focal point of the second reflector, and the curved surface is disposed substantially between the center and the second reflector to collect the electromagnetic radiation and transmit it to the tapered light pipe.
Electromagnetic radiation emitted by a source of electromagnetic radiation is collected and focused onto a target by positioning the source of electromagnetic radiation at a focal point of a first reflector so that the first reflector produces rays of radiation reflected from the first reflector that converge substantially at a focal point of the second reflector. A substantially hemispherical or toroidal optical coupling element is positioned so that a center of the optical coupling element is substantially proximate with the focal point of the second reflector, whereby the converging rays of radiation reflected from the second reflector pass through a substantially curved surface of the optical coupling element and toward the focal point of the second reflector.
The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.